In an NC (Numerical Control) machine tool, contouring working may be used in the case where cutting working of a complete round is applied to a workpiece. The contouring working is working in which a grindstone attached to a main shaft is inserted into a hole of the workpiece and cutting is applied to an inside wall of the hole by moving the grindstone along the outline of the hole while rotating the grindstone.
Conventionally, whether or not the cut hole is a complete round has been determined by measuring a roundness error using a ballbar type measuring arm shown in Japanese Patent Application Laid-Open No. 61-209857 (FIG. 1), or by measuring the roundness error by a measurement device as shown in Japanese Patent Application Laid-Open No. 4-240051 (FIG. 1). This error includes a position error caused by elastic change of a feed shaft drive system with change in feed speed and feed direction of a machine tool, a position error caused by rolling/pitching/yawing motion of a moving body such as a slide carrying a main shaft, and a position error caused by follow-up delay of a servomotor.
Although rigidity of the feed shaft drive system has been increased or the follow-up delay of the servomotor system has been solved in order to correct these position errors conventionally, it has been difficult to assure accuracy of a complete round in these methods. Also, it has been very difficult to solve the position error caused by rolling/pitching/yawing motion of the moving body such as the slide carrying the main shaft.